Heat a large cast iron skillet over medium-high heat. Add butter and let it melt until foaming.
Add corn kernels in an even layer. Let char undisturbed for 3 minutes, then stir and cook for 2 more minutes until golden brown spots appear.
Add garlic and jalapeno to the pan. Cook for 1 minute until fragrant. Transfer mixture to a large bowl and let cool for 5 minutes.
In a small bowl, whisk together mayonnaise, sour cream, lime juice, chili powder, smoked paprika, and cumin until smooth.
Pour the dressing over the cooled corn. Add cotija cheese, cilantro, red onion, and green onions (reserve some cheese and cilantro for topping). Fold together until evenly coated.
Taste and adjust seasoning with salt, pepper, or lime juice. Transfer to serving bowl and top with reserved cotija and cilantro. Sprinkle with extra chili powder before serving.